1. Technical Field of the Invention
The present invention relates to reversible heat-sensitive paper and methods for writing information on it.
2. Description of Prior Art
A sheet of conventional heat-sensitive recording paper is composed of a supporting base (for example, a paper sheet) provided with a heat-sensitive recording layer on the base, and when the sheet is heated by a heating head, heating pen, laser light, etc., information on image, barcode, etc. is written on the heat sensitive recording layer. However, such heat-sensitive recording paper has a disadvantage in that once information is written, the information cannot be erased, so the paper cannot be used again.
To solve this problem, heat-sensitive recording materials with a reversible property were invented and patent applied (for instance, laid-open Japanese patent No. 179043/1995). The reversible heat-sensitive recording material according to the laid-open Japanese patent No. 179043/1995 normally contains a colorless or light-colored electron donative dyestuff precursor, and a special electron acceptive compound that reversibly changes the color of the dyestuff precursor when the compound is heated, and the material can create and erase an image with a high contrast and can maintain images stably over a wide range of erasing temperatures, as advantageous characteristics.
One of the reversible heat-sensitive papers developed using such a reversible heat-sensitive recording material is, for example, leuco-based rewritable heat-sensitive paper using an electron donative dyestuff precursor. The electron donative dyestuff precursor is shown in FIG. 1; a lactone ring in the molecule opens in an acidic atmosphere and the precursor is colored, and by removing the acidic atmosphere, the lactone ring closes to resume a colorless state. The leuco-based rewritable heat-sensitive paper is combined with the electron donative dyestuff precursor and a reversible developer, where the reversible developer can reversibly create and erase a color through reactions with the electron donative dyestuff precursor. Typical reversible developers include, for example, a phenol-based compound with long chains in an alkyl group.
FIG. 2 shows a coloring and uncoloring model. In FIG. 2, the electron donative dyestuff precursor and the reversible developer in a color-erased state (lower left) are heated, both are fused into a colored state (top), and when they are cooled quickly, the mix is solidified in a near-fusion state, thereby a solid colored state (lower right) is maintained. When the mix is gradually cooled, conversely, the electron donative dyestuff precursor and the reversible developer return to an erased state. Therefore, they can reverse to color or uncolor by quickly or gradually cooling the mix, respectively, after fusion. In addition, a solid mix in a colored state can be transited to the original uncolored state by maintaining the mix for a predetermined time in a temperature range slightly lower than the melting point.
FIG. 3 is a typical sectional view of a reversible heat-sensitive paper 4 used conventionally for coloring and uncoloring with light. In FIG. 3, the numerals represent a base medium by 1, a photo-thermal conversion layer by 2, and a protection film by 3. On the surface of the thermal sensitive paper 1, is formed a reversible heat-sensitive recording layer 1a consisting of an electron donative dyestuff precursor and a reversible developer, formed by a coating method. The opto-thermal conversion layer 2 contains a substance that converts light with a predetermined wavelength into heat, and the substance is normally applied to the surface of the heat-sensitive paper 1, but the substance may also be dispersed in the reversible heat sensitive recording layer. In the opto-thermal conversion layer 2, an organic coloring matter that absorbs selectively light with predetermined wavelengths is normally used. The protection film 3 is a transparent film that protects the surface of the heat-sensitive paper 1 and the opto-thermal conversion layer 2, and is normally composed of a transparent plastic coating.
The above-mentioned conventional reversible heat-sensitive paper 4 (leuco-based rewritable heat-sensitive paper) has the following problems.
(1) The life of an opto-thermal conversion substance used as an opto-thermal conversion layer 2 is so short that if the layer is placed at a bright location for several days (for instance, two or three days), the opto-thermal conversion substance is decomposed, resulting in a shorter life for the reversible heat-sensitive paper. In addition, intense light is required because the reversible heat-sensitive recording layer 2 (an electron donative dyestuff precursor and a reversible developer) must be heated and melted.
(2) When laser light is used to heat the opto-thermal conversion layer 2, a line smaller than the spot diameter of the laser light cannot be written. Consequently, the amount of information is limited when the layer is used to write barcodes, for example.
(3) A conventional writing process is based on so-called raster scanning by scanning lines, therefore, when information spreads two-dimensionally, in the case of letters or two-dimensional barcodes, a long writing time is required.